1. Field of the Invention
The present invention relates generally to capacitive type proximity sensors and more particularly to a capacitive type "capaciflector" proximity sensor which includes a driven shield element behind the sensor.
2. Description of the Prior Art
Capacitive sensors used for proximity sensing and collision avoidance are generally known. However, such sensors do not adequately control stray capacitance and consequently do not perform with an adequate range and sensitivity for many applications such as docking and berthing in outer space.
In U.S. Pat. No. 5,166,679, entitled "Driven Shielding Capacitive Proximity Sensor", John M. Vranish which issued Nov. 24, 1992, the teachings of which are incorporated herein by reference, there is disclosed a capacitive sensing element which uses a capacitively controlled oscillator to drive the sensor element as well as a voltage follower driven shield member located behind the sensor element to reflect energy toward an intruding object to substantially increase range and sensitivity. The intruding object forms an additional capacitive path to ground which in turn alters the frequency of the oscillator. This frequency change is converted to a DC output signal by a frequency to voltage converter.
Such a proximity sensor, however, includes several inherent limitations which include, for example: the central frequency of the oscillator tends to drift as a function of changes in temperature and humidity; the frequency to voltage converter is relatively large in size and consumes a substantial amount of power; and the components and their connections as well as the arrangements in front of a relatively high gain output amplifier presents a source of low frequency drift, and since the signal output is in the low frequency region, this has a tendency to reduce sensor range and sensitivity.